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Issue:
======
This patch is a cumulative patch that includes the functionality of
all security patches released for IIS 4.0 since Windows NT 4.0
Service Pack 6a, and all security patches released to date for IIS
5.0 and 5.1. A complete listing of the patches superseded by this
patch is provided below, in the section titled "Additional
information about this patch". Before applying the patch, system
administrators should take note of the caveats discussed in the
same section.

In addition to including previously released security patches,
this patch also includes fixes for the following newly
discovered security vulnerabilities affecting IIS 4.0, 5.0 and/or
5.1:

- A buffer overrun vulnerability involving the operation of
the chunked encoding transfer mechanism via Active Server
Pages in IIS 4.0 and 5.0. An attacker who exploited this
vulnerability could overrun heap memory on the system, with
the result of either causing the IIS service to fail or
allowing code to be run on the server.
- A Microsoft-discovered vulnerability that is related to the
preceding one, but which lies elsewhere within the ASP data
transfer mechanism. It could be exploited in a similar manner
as the preceding vulnerability, and would havethe same scope.
However, it affects IIS 4.0, 5.0, and 5.1.
- A buffer overrun involving how IIS 4.0, 5.0 and 5.1 process
HTTP header information in certain cases. IIS performs a
safety check prior to parsing the fields in HTTP headers, to
ensure that expected delimiter fields are present and in
reasonable places. However, it is possible to spoof the check,
and convince IIS that the delimiters are present even when they
are not. This flaw could enable an attacker to create an URL
whose HTTP header field values would overrun a buffer used to
process them.
- A Microsoft-discovered buffer overrun vulnerability in IIS 4.0,
5.0 and 5.1 that results from an error in safety check that
is performed during server-side includes. In some cases, a user
request for a web page is properly processed by including the
file into an ASP script and processing it. Prior to processing
the include request, IIS performs an operation on the user-
specified file name, designed to ensure that the file name is
valid and sized appropriately to fit in a static buffer. However,
in some cases it could be possible to provide a bogus, extremely
long file name in a way that would pass the safety check, thereby
resulting in a buffer overrun.
- A buffer overrun affecting the HTR ISAPI extension in IIS 4.0
and 5.0. By sending a series of specially malformed HTR
requests, it could be possible to either cause the IIS service to
fail or, under a very difficult operational scenario, to cause
code to run on the server.
- A denial of service vulnerability involving the way IIS 4.0,
5.0, and 5.1 handle an error condition from ISAPI filters.
At least one ISAPI filter (which ships as part of FrontPage
Server Extensions and ASP.NET), and possibly others, generate
an error when a request is received containing an URL that
exceeds the maximum length set by the filter. In processing
this error, the filter replaces the URL with a null value. A
flaw results because IIS attempts to process the URL in the course
of sending the error message back to the requester, resulting in
an access violation that causes the IIS service to fail.
- A denial of service vulnerability involving the way the FTP
service in IIS 4.0, 5.0 and 5.1 handles a request for the status
of the current FTP session. If an attacker were able to establish
an FTP session with an affected server,and levied a status
request that created a particular error condition, a flaw in the
FTP code would prevent it from correctly reporting the error.
Other code within the FTP service would then attempt to use
uninitialized data, with an access violation as the result. This
would result in the disruption of not only FTP services, but also
of web services.
- A trio of Cross-Site Scripting (CSS) vulnerabilities affecting
IIS 4.0, 5.0 and 5.1: one involving the results page that's
returned when searching the IIS Help Files, one involving HTTP
error pages; and one involving the error message that's returned
to advise that a requested URL has been redirected. All of these
vulnerabilities have the same scope and effect: an attacker who
was able to lure a user into clicking a link on his web site
could relay a request containing script to a third-party web
site running IIS, thereby causing the third-party site's response
(still including the script) to be sent to the user. The script
would then render using the security settings of the third-party
site rather than the attacker's.

Mitigating Factors:
====================
Buffer overrun in Chunked Encoding transfer:
- On default installations of IIS 5.0 and 5.1, exploiting the
vulnerability to run code would grant the attacker the privileges
of the IWAM_computername account, which has only the privileges
commensurate with those of an interactively logged-on
unprivileged user.
- The vulnerability requires that Active Server Pages (ASP) be
enabled on the system in order to be exploited. Version 1.0 of
the IIS Lockdown Tool removes ASP by default, and the current
version (version 2.1) removes it by default if Static Web Server
has been selected.
- The URLScan tool can be configured to prevent chunked encoding
requests. If this has been done, the vulnerability could not be
exploited.

Microsoft-discovered variant of Chunked Encoding buffer overrun:
- This vulnerability is subject to exactly the same mitigating
factors as the buffer overrun in the Chunked Encoding transfer,
with one exception. The URLScan tool could not be used to protect
against the vulnerability.

Buffer Overrun in HTTP header handling:
- On default installations of IIS 5.0 and 5.1, exploiting the
vulnerability to run code would grant the attacker the
privileges of the IWAM_computername account, which has only
the privileges commensurate with those of an interactively
logged-on unprivileged user.
- The vulnerability requires that Active Server Pages (ASP) be
enabled on the systemin order to be exploited. Version 1.0
of the IIS Lockdown Tool removes ASP by default, and the
current version (version 2.1) removes it by default if
Static Web Server has been selected.
- The URLScan tool's default ruleset would likely limit the
attacker to using this vulnerability for denial of service
attacks only.

Buffer Overrun in ASP Server-Side Include Function:
- On default installations of IIS 5.0 and 5.1, exploiting the
vulnerability to run code would grant the attacker the privileges
of the IWAM_computername account, which has only the privileges
commensurate with those of an interactively logged-on user.
- The vulnerability requires that Active Server Pages (ASP) be
enabled on the system in order to be exploited. Version 1.0
of the IIS Lockdown Tool removes ASP by default, and the current
version (version 2.1) removes it by default if Static Web Server
has been selected.
- The URLScan tool's default ruleset would likely limit the
attacker to using this vulnerability for denial of service
attacks only.

Buffer overrun in HTR ISAPI extension:
- Microsoft has long recommended disabling the HTR ISAPI extension.
Systems on which this has been done would be at no risk from the
vulnerability. (All versions of the IIS Lockdown Tool disable HTR
support by default).
- The URLScan tool, if using its default ruleset, would prevent
this vulnerability from being exploited to run code on the server
even if HTR support was enabled.
- The vulnerability could only be used to run code on the server if
the attacker knew the locations of certain information in memory.
In practice, the most likely such situation would occur if the
web server had never served any web content since being rebooted.
In all other cases, it would only be possible to use the
vulnerability for denial of service attacks.
- On default installations of IIS 5.0 and 5.1, exploiting the
vulnerability to run code would grant the attacker the privileges
of the IWAM_computername account, which has only the privileges
commensurate with those of an interactively logged-on user.
- If the vulnerability were used in a denial of service attack,
normal operation could be restored on an IIS 4.0 server by
restarting the IIS service; on IIS 5.0 and higher, the service
would automatically restart itself.

Access violation in URL error handling:
- An IIS 4.0 server could be put back into normal operation by
restarting the service. An IIS 5.0 or 5.1 server would
automatically restart the service.
- The vulnerability could only be used for denial of service
attacks. There is no capability to use the vulnerability to gain
privileges on the system.
- The sole ISAPI filter known to generate the error that results in
the access violation ships only as part of FrontPage Server
Extensions and ASP.NET. ASP.NET is not installed by default, and
FPSE can be uninstalled if desired.

Denial of service via FTP Status request:
- The IIS Lockdown Tool disables FTP support by default.
- An IIS 4.0 server could be put back into normal operation by
restarting the service. An IIS 5.0 or 5.1 server would
automatically restart the service.
- The vulnerability could only be used for denial of service
attacks. There is no capability to use the vulnerability to gain
privileges on the system.

Cross-site Scripting in IIS Help File search facility, HTTP Error
Page, and Redirect Response message:
- The vulnerabilities could only be exploited if the attacker could
entice another user into visiting a web page and clicking a link
on it, or opening an HTML mail.
- The Redirect Response vulnerability could only be exploited if
the user was running a browser other than Internet Explorer. IE
does not actually render the text in the Redirect Response, but
instead recognizes it by its response header and processes the
redirect without displaying any text.